Epoxy compounds and isocyanate compounds are known to react to form oxazolidones. The reaction is also known to form substantial quantities of undesirable by-products. A major by-product results from the trimerization of isocyanates to isocyanurate, allophanate and biuret compounds, which are highly functional and typically give rise to highly crosslinked and brittle polymers. A reaction product free of such impurity exhibits improved thermal and mechanical properties compared to one containing a higher proportion of isocyanurate compounds.
Oxazolidones are heterocyclic ring systems containing nitrogen and oxygen functionality, which can be generically represented by the following formula (I): ##STR1## Most conventional catalysts used in preparing such compounds do not efficiently and cleanly drive the reaction to completion.
In large part, the catalysts available in the prior art are of low activity, are generally difficult and unsafe to handle, are not available in commercial quantities, are used only in solvent, or are otherwise unacceptable for a commercial setting. Some also promote epoxy polyether formation.
Ashida, U.S. Pat. No. 4,066,628 teaches the use of a catalyst selected from the group consisting of ZnR.sub.2, Zn(OCOR).sub.2, ZnX and AlR.sub.3 wherein R is an alkyl group having 1 to 12 carbon atoms and X is an organic bidentate group. The preparation of oxazolidones from an organic isocyanate and an epoxide using a quaternary ammonium halide and an alcohol as cocatalysts is disclosed in Schramm; U.S. Pat. No. 3,313,747. Zinc chloride and ferric chloride have been proposed as catalysts for the foregoing reaction [K. Gulbins and K. Hamann, Angew. Chem., 70, 705 (1958)]. Zinc bromide has been proposed as a catalyst for the reaction of an epoxide, 3-phenoxy-1,2-propylene, and phenylisocyanate in dimethylformamide, with only 33% yield of the oxazolidone [Sandler, J. Polymer Science, Part A-1, 5, 1481 (1967)]. Lithium chloride has been found by K. Gulbins to be a catalyst for the reaction between an aromatic isocyanate and an aromatic epoxide [Chem. Ber., 93, 1875, (1960)]. Lithium n-butoxide has been found by R. R. Deleone to be an oxazolidone catalyst [J. Polymer Science, Part A-1, 8, 609, (1970)]. An adduct of lithium bromide and tributyl phosphine oxide has been reported by J. E. Herweh and W. J. Kauffman to be an oxazolidone catalyst [Tetrahedron Letters, no. 12, 809, (1971)]. Glasgow, U.S. Pat. No. 3,702,839 (Nov. 14, 1972) discloses that a phosphonium salt is an oxazolidone catalyst. S. Kimura and H. Samejima (Japanese Patent Public Disclosure 48-70797, September 25, 1973) disclose that monomer-soluble catalysts selected from the group consisting of specific organic metal chelates, alkyl glycidyl ammonium salts, trialkyl (triaryl or tricycloalkyl) borates, and trialkyl (or triaryl) phosphines are oxazolidone catalysts. Y. Iseda and F. Odaka et al. (Japanese Patent Public Disclosure 49-37999, Apr. 9, 1974) disclose that epoxy-soluble quaternary ammonium compounds and/or lithium halidephosphine oxide complexes are oxazolidone catalysts. Ashida, U.S. Pat. No. 3,817,938 (Jun. 18, 1974) discloses that alkoxides or phenoxides of metals of Group IIA or IIIA of the Periodic Table are oxazolidone catalysts. M. J. Marks et al., U.S. Pat. No. 4,658,007 (Apr. 14, 1987) disclose the preparation of an oxazolidone-containing polymer or precursor from a polyepoxide and a polyisocyanate using an organoantimony iodide catalyst. We are also aware of DiSalvo, U.S. Pat. No. 4,424,316; Goel, U.S. Pat. No. 4,613,660; and Waterman, U.S. Pat. No. 4,731,428, each of which relates to the reaction of epoxy compounds and isocyanate compounds in the preparation of curing materials. Standard catalyst systems do not efficiently drive the oxazolidone forming reaction. Further, standard isocyanates such as MDI and TDI form unwanted reaction by-products. In view of these facts, a substantial need exists for a reaction system including isocyanate reactants and catalyst that cooperate to smoothly form oxazolidone without substantial formation of reaction by-product.